Existing subscriber loops can readily provide two-way digital transmission (full-duplex) on a pair of wires using analog signals at voice-band frequencies. This is achieved by amplitude-shift keying, phase-shift keying, frequency-shift keying, or other such techniques. However, full-duplex transmission of high-speed digital signals at ultrasonic bit rates is difficult to achieve on a single communication path. It has been proposed therefore to employ a time compression multiplex (TCM) technique on a half-duplex transmission system wherein a burst-mode or ping-pong approach is utilized.
Typically in such TCM systems, the digital information signal to be transmitted is divided into discrete portions and each portion compressed with respect to time to form a so-called "burst", occupying less than one-half the time of the original portion. The transmitter at each terminal alternately transmits the burst onto the path, following which the associated receiver at each terminal can receive a corresponding burst from the other transmitter. On receipt, each burst is expanded to occupy its original time span. Externally, the system appears to be transmitting the two digital information streams continuously and simultaneously, i.e. full-duplex communication. So far as the transmission path is concerned, however, half-duplex transmission takes place with alternate bursts travelling in opposite directions.
Having transmitted its own burst, each transmitter must wait until the incoming burst from the other transmitter has been cleared from the communcation path before it can transmit again. Arrival of the incoming burst will be delayed by at least a time interval equal to twice the transmission delay or propagation time of the path. The time interval (dead time) detracts from the efficiency of utilization of the communication path. Thus, for a given burst length, the efficiency decreases as the path length increases. The efficiency can be improved, for a given path length, by increasing the length of each burst, thus increasing the "on" time relative to the "dead" time. However, this exacerbates the synchronizing timing problem by increasing the corresponding reception interval during which the receiver is turned off and hence the receiver's clock receives no control bits to keep it synchronized. As a result, these systems function well on short loops, particularly with short bursts, in which strong signals are received. However, on long loops spurious signals resulting from cable irregularities such as gauge changes and bridge taps (which cause reflected pulses), can cause false synchronization to be established.
In a paper by R. Montemurro et al entitled "Realisation d'un equipment terminal numerique d'abonne pour service telephonique et de donnees", colloque international de commutation; International Switching Symposium, Paris, 11 May 1979, pp. 926-933; there is described a synchronization technique in which two frame bits are added, one at the beginning and the other at the end of each burst. This arrangement helps to prevent false synchronization since it can only occur if one or the other of the bits which was erroneously detected as a true synchronization bit, is outside the burst. Thus, essentially the only condition that can cause false synchronization to be detected is one in which the two detected bits, one a spurious bit and the other a signal bit, have the correct polarity and are spaced from one another by the correct interval. However, such a system utilizes a guard time to insure that adequate delay of all reflected signals takes place before signal transmission commences in the opposite direction.
This problem has been alleviated by providing a window which is coextensive with the received burst once synchronization is established. Such a technique has been described in applicant's U.S. Pat. No. 4,476,558 issued Oct. 9, 1984 to Ephraim Arnon. Thus, once frame synchronization has been established, the signals are only gated to the receiver during a window interval which is coextensive with that of the received bursts. With this arrangement a signal burst can be transmitted immediately after one is received at a slave station, with no guard time between the two bursts. However, a problem arises at a master or control station due to the relatively large capacitance of the line. It was found that this capacitance can cause post transmission transients resulting in a trailing edge on each of the transmitted bursts. This trailing edge may be detected as an initial synchronization bit which in conjunction with some of the received signal bits could cause the circuit to repeatedly jump into and out of a false synchronization mode, thereby preventing true synchronization from being established.
This problem has been further alleviated by utilizing the final synchronization bit of one burst and the initial synchronization bit of the following burst to establish frame synchronization as described in applicant's U.S. Pat. No. 4,467,473 issued Aug. 21, 1984 to Ephraim Arnon et al. This technique is possible since the bursts are transmitted at regular intervals under control of the master station. As described in this Patent there is a check for the presence of an initial synchronization bit a preselected number of bit periods following the final synchronization bit of the previous frame, rather than the presence of initial and final synchronization bits in the same frame.